1. Field of the Invention
The present invention relates to current mirror circuits generally and to methods of driving such circuits, and more particularly to a current mirror circuit capable of supplying a desired current that corresponds to a first current regardless of the threshold voltages of one or more transistors included in a driving circuit that contains the current mirror circuit. The invention further relates to a method of driving the driving circuit that contains a current mirror circuit constructed in accordance with the principles of the invention.
2. Discussion of Related Art
A current mirror circuit is a circuit in which a value of an output current is determined by a value of an input current. The current mirror circuit is used for various circuits.
FIG. 1 illustrates a conventional current mirror circuit that includes a first transistor M1 and a second transistor M2. Each transistor M1 and M2 includes a first terminal, a second terminal, and a gate terminal.
The first terminal of the transistor M1 is electrically connected to a voltage source VDD. The second terminal and the gate terminal of the transistor M1 are electrically connected to each other. That is, electric current flows through the transistor M1 so that the transistor M1 not only serves as a diode, but also supplies to its second terminal a current I1 that corresponds to the voltage source VDD. The first terminal may be set as one of a source terminal and a drain terminal and the second terminal may be set to be different from the first terminal. For example, when the first terminal is set as the source terminal, the second terminal is set as the drain terminal.
The transistor M2 is electrically connected to the transistor M1 to form a current mirror circuit. Therefore, the gate terminal of the transistor M2 is electrically connected to the gate terminal of the transistor M1, and the first terminal of the transistor M2 is electrically connected to the voltage source VDD. Assuming that a ratio of a channel width and a channel length (W/L) of the transistor M1 approximately equals a ratio of a channel width and a channel length (W/L) of the transistor M2, a current I2 flowing from the transistor M2 is preferably set to equal the current I1 that flows from the transistor M1. That is, the current mirror circuit controls the first current I1 to thereby control the value of the second current I2 that is supplied to one or more external components.
The conventional current mirror circuit may have several applications. For example, the conventional current mirror circuit may be electrically connected to a driver 2 and used as a bias unit 1 as illustrated in FIG. 2. In operation, the bias unit 1, which includes the current mirror circuit of FIG. 1, supplies the second current I2 (that is, bias current) to the driver 2 so that the driver 2 can perform desired operations. In turn, the driver 2 performs a predetermined operation when the second current I2 is supplied. By way of example, the driver 2 may function as either an amplifier or a buffer.
In the conventional current mirror circuit, the difference between a threshold voltage of the transistor M1 and a threshold voltage of the transistor M2 creates a difference in current outputted from each transistor. Therefore, it is not possible to generate a desired current outputted from the transistor M2 that matches the current outputted from the transistor M1. A threshold voltage may be defined as a gate voltage at or below which the transistor remains turned off (and no current flows) and above which the transistor turns on (and current flows).
Otherwise identical transistors M1 and M2 may have different threshold voltages due to deviations caused by manufacturing or other processes. For example, the threshold voltage of the transistor M1 may be set as 0.7V and the threshold voltage of the transistor M2 may be set as 0.3V. In such a case, the first current I1 that flows via the transistor M1 and the second current I2 that flows via the transistor M2 will have different values, each of which corresponds to one of the different threshold voltages. Due to the different threshold voltages, it is not possible to generate a desired second current I2. This is particularly problematic when the current mirror circuit is used for a part sensitive to current such as a pixel of a light emitting display, since any deviation in current generated by the different threshold voltages of the transistors M1 and M2 causes serious problems.